Semiconductor chip and gas sensor comprising such a semiconductor chip

Abstract

The invention relates to a semiconductor chip (100) for determining a thermal conductivity, the thermal diffusivity, the specific heat capacity and secondly a density and / or a viscosity of a gas mixture, comprising: a main body (110); a cantilever cavity (120) surrounded by the main body; an oscillator cantilever (130), which projects into the cantilever cavity (120), and a piezoelectric exciter (131, 132, 133) for exciting vibrations; at least one heater cantilever (140), which projects into the cantilever cavity (120); a heating element (144), in a cantilevered end portion (142); and a temperature sensor element (154, 164); wherein a portion of the cantilever cavity (120) extends between the heating element (144) and the temperature sensor element (154, 164), and wherein at least one portion of the cantilever cavity (120) extends between the heater cantilever (140) and the oscillator cantilever (130); wherein the main body (110) also has a main body web (112) which extends between an oscillator region (123) and a heater region (124) of the cantilever cavity (120), wherein the oscillator cantilever (130) is arranged in the oscillator region (123), and the heater cantilever (140) is arranged in the heater region (124).

Description

[0001] Semiconductor chip and gas sensor with such a semiconductor chip

[0002] The present invention relates to a semiconductor chip and a gas sensor comprising such a semiconductor chip, in particular a semiconductor chip for determining firstly a thermal conductivity, a thermal diffusivity and / or the specific heat capacity of a gas mixture and secondly a density and / or a viscosity of the gas mixture.

[0003] A generic semiconductor chip and a generic gas sensor are disclosed in the publication WO 2022 / 008212 A1. The semiconductor chip described therein comprises: a base body; a cantilever cavity surrounded by the base body; at least one oscillator cantilever, which cantilevers from the base body into the cantilever cavity and has at least one piezoelectric exciter for exciting cantilever oscillations; at least one heater cantilever, which cantilevers from the base body into the cantilever cavity and has a heating element in a cantilevered end portion facing away from the base body; at least one first temperature sensor element; wherein at least a portion of the cantilever cavity extends between the heating element and the first temperature sensor element; and wherein at least a portion of the cantilever cavity extends between the heater cantilever and the oscillator cantilever.

[0004] Investigations in connection with the present invention have shown that the gas density in the vicinity of the oscillator cantilever exhibits a gradient due to the thermal measurements. This can impair the accuracy of the density or viscosity measurement. To remedy this, alternating measurement mode for determining the thermal measured variables and the measurements with the oscillator cantilever is considered, but this significantly increases the time required for a measurement cycle. It is therefore the object of the present invention to remedy this.

[0005] The semiconductor chip according to the invention serves to determine, firstly, a thermal conductivity, a thermal diffusivity and / or the specific heat capacity of a gas mixture and, secondly, a density and / or a viscosity of the gas mixture, wherein it comprises: a base body; a cantilever cavity surrounded by the base body; at least one oscillator cantilever, which cantilevers from the base body into the cantilever cavity and has at least one piezoelectric exciter for exciting cantilever oscillations; at least one heater cantilever, which cantilevers from the base body into the cantilever cavity and has a heating element in a cantilevered end section facing away from the base body; at least one first temperature sensor element; wherein at least a section of the cantilever cavity runs between the heating element and the first temperature sensor element;and wherein at least a portion of the cantilever cavity extends between the heater cantilever and the oscillator cantilever; wherein, according to the invention, the base body further comprises a base body web that extends between an oscillator region and a heater region of the cantilever cavity, wherein the oscillator cantilever is arranged in the oscillator region, and the heater cantilever is arranged in the heater region.

[0006] The semiconductor chip according to the invention has at least one means for detecting the temperature of the heater cantilever. In the simplest case, this means is the heating element, with the temperature of the heater cantilever being determined based on a measured value of the electrical resistance of the heating element. Furthermore, the heater cantilever can have a separate sensor element for temperature measurement.

[0007] In a further development of the invention, the semiconductor chip comprises at least one sensor cantilever carrier; which cantilevers from the base body into the cantilever carrier cavity and has the first temperature sensor element in a cantilevered end section facing away from the base body, wherein the sensor cantilever carrier is arranged in the heater region.

[0008] In a further development of the invention, the oscillator cantilever beam has a second temperature sensor element, in particular in a cantilevered end section of the oscillator cantilever beam facing away from the base body.

[0009] In a further development of the invention, the oscillator cantilever is spaced from the heater cantilever by not less than 500 pm, for example not less than 800 pm; and / or the oscillator cantilever is spaced from the heater cantilever by not more than 2000 pm, for example not more than 1500 pm.

[0010] In a further development of the invention, the oscillator cantilever beam has a thickness of not less than 3 pm, in particular not more than 20 pm, and / or the oscillator cantilever beam has a length of not less than 400 pm, and / or the oscillator cantilever beam has a length of not more than 1000 pm, in particular not more than 750 pm.

[0011] In a further development of the invention, the chip comprises silicon, in particular polysilicon.

[0012] In a further development of the invention, the semiconductor chip further comprises: a hydrogen-tight coating, which in particular comprises Al2O3 or ALN, in particular for protecting metallic conductor tracks that are prepared on surface sections of the base body or the cantilever beams, so that the coating in particular covers the conductor tracks.

[0013] In a further development of the invention, the heater cantilever beam has a thermal resistance such that in a carbon dioxide atmosphere under reference conditions, a temperature difference between the cantilevered end of the heater cantilever beam and the base body-side end of the cantilever beam per heating power is not less than 5 K / mW, in particular not less than 6 K / mW.In a further development of the invention, the cantilever beams each have a longitudinal direction in which they are cantilevered, wherein the cantilever beams each have a transverse direction perpendicular to their longitudinal direction in a plane of the surface of the cantilever beams, wherein the heater cantilever beam has a greater width in the transverse direction in its cantilevered end section, in which the heating element is arranged, than in its base section facing the main body, wherein in particular the heating element has a greater width in the transverse direction than in its base section facing the main body, wherein the width of the cantilevered end section is, for example, at least one and a half times, in particular at least 1.9 times the width of the base section facing the main body.

[0014] In a further development of the invention, the first temperature sensor element is not less than 50 pm away from the heating element, and / or the first temperature sensor element is not more than 200 pm away from the heating element.

[0015] In a further development of the invention, the base body has a substantially polygonal profile around the cantilever cavity, wherein the at least one sensor cantilever and the heater cantilever project from different sides, in particular opposite sides of the polygonal profile, from the base body into the cantilever cavity.

[0016] In a further development of the invention, the semiconductor chip further comprises at least one cap covering the cantilever cavity. In one embodiment of this further development, the cap has a cover surface facing the cantilever cavity and at least one projection protruding from the cover surface in the direction of the base body web.

[0017] In a further development of the invention, an orthogonal projection of the projection onto a plane representing the cover surface overlaps with an orthogonal projection of the base body web onto this plane by at least 50%, in particular by at least 75% and preferably by 100%.

[0018] In a further development of the invention, the distance between the base body web 112 and the projection is not more than 75%, in particular not more than 50% of the distance between the oscillator cantilever beam and the plane representing the cover surface.

[0019] In a further development of the invention, the hood comprises a porous material to enable a diffusive gas exchange through the hood.

[0020] The gas sensor according to the invention for analyzing a gas or gas mixture comprises a semiconductor chip according to one of the preceding claims; and a measuring and operating circuit configured to determine the density and viscosity of the gas mixture based on vibration properties of the oscillator cantilever beam, and to determine the thermal conductivity, the thermal diffusivity, and / or the specific heat capacity of the gas mixture based on temperature measurement values ​​of the temperature sensor element and the power consumption of the heating element.

[0021] The invention will now be explained in more detail with reference to the embodiment shown in the drawings.

[0022] It shows:

[0023] Fig. 1: a plan view of an embodiment of the semiconductor chip according to the invention in the plane of the cantilever beams;

[0024] Fig. 2: a cross section through an embodiment of the semiconductor chip according to the invention along the line (ll-ll) in Fig. 1;

[0025] Fig. 3: a detailed plan view of a part of the oscillator region of the cantilever cavity and an adjacent region of the base body of the exemplary embodiment of a semiconductor chip according to the invention from Figs. 1 and 2; and

[0026] Fig. 4: a detail of the heater cantilever beam of the embodiment shown in Figs. 1 to 3; and

[0027] Fig. 5: a schematic representation of the layer structure of an embodiment of the semiconductor chip.

[0028] The exemplary embodiment of a semiconductor chip 100 according to the invention shown in Figures 1 to 4 comprises a base body 110, which in particular comprises polysilicon. Polysilicon has a lower thermal conductivity than monosilicon, which reduces heat losses in heating elements.

[0029] The base body 110 surrounds a cantilever cavity 120, which is divided by a base body web 112 into an oscillator region 123 and a heater region 124. The area of ​​the heater region 124 is approximately 0.5 mm 2 It is approximately twice the size of the area of ​​the oscillator region 123. An oscillator cantilever beam 130 extends into the oscillator region 123, having a length of approximately half a millimeter and a width of approximately one-tenth of a millimeter. The oscillator cantilever beam 130 has, in its cantilevered end section, a temperature sensor element 134 which—as shown in Fig. 3—in particular comprises a Pt resistance element. The oscillator cantilever beam 130 further has piezoelectric transducers 131, 132, 133 for exciting or detecting oscillations of the oscillator cantilever beam 130. The piezoelectric transducers can, for example, comprise AlN, in particular AlN doped with Sc.

[0030] The thickness of the oscillator cantilever beam 130, for example, is approximately 5 μm. This thickness is very suitable for gas pressures up to several MPa. For higher gas pressures, greater material thicknesses should be considered. For a gas pressure of, say, 100 MPa, a material thickness of approximately 10 μm is advantageous.

[0031] In the heater region 124, which is spatially separated from the oscillator region 123, a heater cantilever beam 140 is arranged, which projects with a length of approximately half a millimeter from the base body 110 into the cantilever beam cavity 120. The heater cantilever beam 140 has a heating element 144 in its cantilevered end section 142, which in particular comprises a meandering resistance structure, for example made of Pt. The cantilevered end section 142 with the heating element 144 has a width B of approximately one-tenth of a millimeter and a length L of approximately one-tenth of a millimeter to approximately one-sixth of a millimeter, wherein the heater cantilever beam 140 has a base section 146 with a width of only approximately one-twentieth of a millimeter between the cantilevered end section and the base body. This reduced width of the base section 146 compared to the cantilevered end section has the effect that the thermal resistance of the heater cantilever is increased.This allows a larger temperature difference to be achieved with a given heating power compared to the surroundings of the heating element 144. For example, with a heating power of 3 mW in a carbon dioxide environment, a temperature difference of approximately 62 K is achieved, whereas with a design with the full width of the heater cantilever beam 140 of one-tenth of a millimeter, only a temperature difference of approximately 37 K was achieved with this heating power. The use of polysilicon as the material of the semiconductor chip 100 also makes it easier to achieve a sufficiently large temperature difference, since polysilicon has a lower thermal conductivity than monocrystalline silicon. The heater cantilever beam 140 further has a temperature sensor element 148 in its cantilevered end section 142 for detecting the temperature of the cantilevered end section. This temperature sensor element 142 comprises, in particular, a Pt resistance element.In principle, the temperature can also be determined based on the resistance value of the heating element, but an independent temperature measurement with the additional temperature sensor element 148 is more accurate.

[0032] Laterally spaced from the heater cantilever 140, two sensor cantilevers 150, 160 project into the heater region 124 of the cantilever cavity 120. The sensor cantilevers 150, 160 on the one hand and the heater cantilever 140 on the other hand are connected to the heater region 124 on opposite sides of the opening in the base body 110 in order to thermally decouple the heater cantilever 140 from the sensor cantilevers 150, 160 as much as possible. The sensor cantilever beams 150, 160 each have a temperature sensor element 154, 164 in their cantilevered end sections, which in particular has a Pt resistance element with a similar structure to the resistance element 134 on the oscillator cantilever beam 130. The temperature sensor elements 154, 164 have different distances from the heating element, which are between approximately one twentieth of a millimeter and approximately one third of a millimeter, wherein one of the sensor elements is not more than one fifth of a millimeter away from the heating element.With the sensor elements at different distances, the measuring range for thermal conductivities of a surrounding gas mixture is increased compared to arrangements with only one sensor element.

[0033] The oscillator cantilever 130 is spaced approximately 1200 μm from the heater cantilever 140 to minimize temperature gradients along the oscillator cantilever caused by the heating element. The base body web 112, which forms a thermal bridge running parallel to the oscillator cantilever 130, also contributes to reducing temperature gradients.

[0034] The temperature fluctuations along the oscillator cantilever beam 130 are therefore no more than 0.02 K when the heating element is heated by 62 K.

[0035] The semiconductor chip 100 further comprises a cap 180 covering the cantilever cavity 120. The cap comprises, in particular, a porous material through which gas exchange can occur. The pore size can be, for example, approximately 1 μm. Suitable materials include sintered bronze, silicon, ceramic, and polymers. The wall thickness of the cap 180 is approximately 0.5 to 0.8 mm.

[0036] In the exemplary embodiment shown here, a projection 184 with a prismatic cross-section protrudes from an end face 182 of the hood opposite the cantilever cavity 120 and runs parallel to the base body web 112. This projection 184 serves as a thermal bridge and thus, together with the base body web, contributes to the homogenization of the temperature in the oscillator region 123.

[0037] The base body 110 can have a first base-side temperature sensor element 174 adjacent to the oscillator region 123 of the cantilever cavity 120, which also includes a Pt resistance element. However, this is entirely optional and can be used, for example, to determine temperature gradients across the oscillator region, for example by comparing its temperature measurement value with a temperature measurement value of the temperature sensor element 134 of the oscillator cantilever 130. A second optional base-side temperature sensor element 172 can be prepared on the base body web 112, wherein this has a similar structure to the first base-side temperature sensor element 174. The second base-side temperature sensor element 172 on the base body web 112 can be used, together with the first base-side temperature sensor element, to monitor the temperature homogeneity in the oscillator region 123.

[0038] Both the piezoelectric electrodes 131, 132, 133 and the resistance elements of the temperature sensor elements 134, 148, 174, and the heater 144 are connected to a measuring and operating circuit via metallic conductor tracks 180, which are prepared on a surface 126 of the base body 110. The semiconductor chip 100 has a layer structure, which is briefly explained using the oscillator cantilever beam 130 shown in Fig. 5. The layer structure initially comprises a first silicon dioxide layer 24 on a substrate body 22 made of silicon. A polysilicon layer 26 with a thickness of approximately 5 pm is prepared on the first silicon dioxide layer 24. The cantilever beams 130 are prepared in this polysilicon layer 26. The polysilicon layer 26 is followed by a second silicon dioxide layer 28.Furthermore, an adhesion promoter layer 30 is provided, on which first platinum structures 32 are prepared, which, on the one hand, comprise a resistance element of the temperature sensor element 134 and an optional heater 136 on the oscillator cantilever 130, and, on the other hand, electrodes of the piezoelectric transducers 131, 132, 133. Ti, along with AlN, and Al2O3 are particularly suitable as the adhesion promoter layer 30, the latter being preferred because they have better compatibility with hydrogen. The platinum structures 32 are followed by an AlN layer 34 for the construction of the piezoelectric transducers 131, 132, 133 and second platinum structures 36, which comprise electrodes of the piezoelectric transducers and an optional heater 136 for the oscillator cantilever 130.The conductor tracks, electrodes of the piezoelectric transducers 131, 132, 133, and PT resistance elements are covered with a protective layer 38, which comprises, for example, AlN or Al2O3, and is particularly diffusion-tight against hydrogen. The layer structure is similar for the other cantilever beams.

Claims

Patent claims 1. A semiconductor chip (100) for determining, firstly, a thermal conductivity, a thermal diffusivity, and / or a specific heat capacity of a gas mixture, and secondly, a density and / or a viscosity of the gas mixture, comprising: a base body (110); a cantilever cavity (120) surrounded by the base body; at least one oscillator cantilever (130) projecting from the base body (110) into the cantilever cavity (120) and having at least one piezoelectric exciter (131, 132, 133) for exciting cantilever oscillations; at least one heater cantilever (140) projecting from the base body (110) into the cantilever cavity (120) and having a heating element (144) in a projecting end portion (142) facing away from the base body (110); at least one first temperature sensor element (154, 164); wherein at least a portion of the cantilever cavity (120) extends between the heating element (144) and the first temperature sensor element (154, 164);and wherein at least a portion of the cantilever cavity (120) extends between the heater cantilever (140) and the oscillator cantilever (130); characterized in that the base body (110) further comprises a base body web (112) which extends between an oscillator region (123) and a heater region (124) of the cantilever cavity (120), wherein the oscillator cantilever (130) is arranged in the oscillator region (123), and the heater cantilever (140) is arranged in the heater region (124).

2. The semiconductor chip according to claim 1, further comprising at least one sensor cantilever carrier; which cantilevers from the base body into the cantilever carrier cavity and has the first temperature sensor element in a cantilevered end portion facing away from the base body, wherein the sensor cantilever carrier is arranged in the heater region.

3. Semiconductor chip according to one of the preceding claims, wherein the oscillator cantilever carrier has a second temperature sensor element, in particular in a cantilevered end section of the oscillator cantilever carrier facing away from the base body.

4. The semiconductor chip according to one of the preceding claims, wherein the oscillator cantilever is spaced from the heater cantilever by no less than 500 pm, for example, no less than 800 pm; and / or wherein the oscillator cantilever is spaced from the heater cantilever by no more than 2000 pm, for example, no more than 1500 pm.

5. Semiconductor chip according to one of the preceding claims, wherein the oscillator cantilever carrier has a thickness of not less than 3 pm, in particular not more than 20 pm, and / or wherein the oscillator cantilever carrier has a length of not less than 400 pm, and / or wherein the oscillator cantilever carrier has a length of not more than 1000 pm, in particular not more than 750 pm.

6. Semiconductor chip according to one of the preceding claims, wherein the chip comprises silicon, in particular polysilicon.

7. Semiconductor chip according to one of the preceding claims, further comprising: a hydrogen-tight coating, which in particular comprises Al2O3 or AlN, in particular for protecting metallic conductor tracks prepared on surface sections of the base body or the cantilever beams.

8. Semiconductor chip according to one of the preceding claims, wherein the heater cantilever has a thermal resistance such that in a carbon dioxide atmosphere under reference conditions a temperature difference between the cantilevered end of the heater cantilever and the base body-side end of the cantilever per heating power is not less than 5 K / mW, in particular not less than 6 K / mW.

9. Semiconductor chip according to one of the preceding claims, wherein the cantilever beams each have a longitudinal direction in which they are cantilevered, wherein the cantilever beams each have a transverse direction perpendicular to their longitudinal direction in a plane of the surface of the cantilever beams, wherein the heater cantilever beam has a greater width in the transverse direction in its cantilevered end section, in which the heating element is arranged, than in its base section facing the main body, wherein in particular the heating element has a greater width in the transverse direction than in its base section facing the main body, wherein the width of the cantilevered end section is, for example, at least one and a half times, in particular at least 1.9 times the width of the base section facing the main body.

10. Semiconductor chip according to one of the preceding claims, wherein the first temperature sensor element is not less than 50 pm away from the heating element, and / or wherein the first temperature sensor element is not more than 200 pm away from the heating element.

11. Semiconductor chip according to claim 2, or one of the preceding claims dependent on claim 2, wherein the base body has a substantially polygonal profile around the cantilever cavity, wherein the at least one sensor cantilever and the heater cantilever project from different sides, in particular opposite sides of the polygonal profile, from the base body into the cantilever cavity.

12. Semiconductor chip according to one of the preceding claims, further comprising: at least one hood covering the cantilever cavity.

13. The semiconductor chip according to claim 12, wherein the hood has a cover surface facing the cantilever cavity and at least one projection which projects from the cover surface in the direction of the base body web.

14. Semiconductor chip according to claim 13, wherein an orthogonal projection of the projection onto a plane representing the cover surface overlaps with an orthogonal projection of the base body web onto this plane by at least 50%, in particular at least 75% and preferably 100%.

15. Semiconductor chip according to claim 14, wherein the distance between the base body web and the projection is not more than 75%, in particular not more than 50%, of the distance between the oscillator cantilever carrier and the plane representing the cover surface.

16. A semiconductor chip according to claim 12 or any claim dependent on claim 12, wherein the hood comprises a porous material to enable diffusive gas exchange through the hood.

17. A gas sensor for analyzing a gas or gas mixture, comprising: a semiconductor chip according to one of the preceding claims; and: a measuring and operating circuit configured to determine the density and viscosity of the gas mixture based on vibration properties of the oscillator cantilever beam, and to determine the thermal conductivity, thermal diffusivity, and / or specific heat capacity of the gas mixture based on temperature measurements of the temperature sensor element and the power consumption of the heating element.

Images